Hybrid orthogonal frequency division multiple access WTRU and method

ABSTRACT

A hybrid orthogonal frequency division multiple access (OFDMA) wireless transmit/receive unit (WTRU) and method are disclosed. A WTRU includes a transmitter and a receiver. The receiver processes received data to recover data mapped to the subcarriers using OFDMA. The receiver recovers first input data by separating user data from multi-user spread data and recovers second input data from non-spread data.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/776,769, filed May 10, 2010, issuing as U.S. Pat. No. 8,023,551 onSep. 20, 2011, which is a continuation of U.S. patent application Ser.No. 11/406,878, filed Apr. 19, 2006, which issued as U.S. Pat. No.7,715,460 on May 11, 2010, which claims the benefit of U.S. ProvisionalApplication No. 60/673,872, filed Apr. 22, 2005, which are incorporatedby reference as if fully set forth.

FIELD OF INVENTION

This application is related to wireless communications.

BACKGROUND

It is expected that future wireless communication systems will providebroadband services such as wireless Internet access to subscribers. Suchbroadband services require reliable and high throughput transmissionsover a wireless channel which is time dispersive and frequencyselective. The wireless channel is subject to limited spectrum andinter-symbol interference (ISI) caused by multipath fading. Orthogonalfrequency division multiplexing (OFDM) and OFDMA are some of the mostpromising solutions for next generation wireless communication systems.

OFDM has a high spectral efficiency since the subcarriers used in theOFDM system overlap in frequency and an adaptive modulation and codingscheme (MCS) may be employed across subcarriers. In addition,implementation of OFDM is very simple because the baseband modulationand demodulation are performed by simple inverse fast Fourier transform(IFFT) and fast Fourier transform (FFT) operations. Other advantages ofthe OFDM system include a simplified receiver structure and excellentrobustness in a multipath environment.

OFDM and OFDMA have been adopted by several wireless/wired communicationstandards, such as digital audio broadcast (DAB), digital audiobroadcast terrestrial (DAB-T), IEEE 802.11a/g, IEEE 802.16, asymmetricdigital subscriber line (ADSL) and is being considered for adoption inthird generation partnership project (3GPP) long term evolution (LTE),cdma2000 evolution, a fourth generation (4G) wireless communicationsystem, IEEE 802.11n, or the like. One key problem with OFDM and OFDMAis that it is difficult to mitigate or control inter-cell interferenceto achieve a frequency reuse factor of one. Frequency hopping andsubcarrier allocation cooperation between cells have been proposed tomitigate inter-cell interference. However, the effectiveness of bothmethods is limited.

SUMMARY

A hybrid orthogonal frequency division multiple access (OFDMA) wirelesstransmit/receive unit (WTRU) and method are disclosed herein. A WTRUincludes a transmitter and a receiver. The receiver processes receiveddata to recover data mapped to the subcarriers using OFDMA, and recoversfirst input data by separating user data from multi-user spread data andsecond input data from non-spread data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary hybrid OFDMA system.

FIG. 2 shows an example of frequency domain spreading and subcarriermapping.

FIG. 3 shows another example of spreading and subcarrier mapping.

FIG. 4 shows an example of time-frequency hopping of subcarriers.

FIG. 5 is a block diagram of an exemplary time-frequency Rake combinerconfigured.

DETAILED DESCRIPTION

Hereafter, the terminology “transmitter” and “receiver” includes but arenot limited to a user equipment (UE), a wireless transmit/receive unit(WTRU), a mobile station, a fixed or mobile subscriber unit, a pager, aNode-B, a base station, a site controller, an access point or any othertype of device capable of operating in a wireless environment.

The features disclosed herein may be incorporated into an integratedcircuit (IC) or be configured in a circuit comprising a multitude ofinterconnecting components.

The teachings herein are applicable to any wireless communication systemthat utilizes OFDMA (or OFDM) and/or code division multiple access(CDMA), such as IEEE 802.11, IEEE 802.16, third generation (3G) cellularsystems, 4G systems, satellite communication systems, or the like.

FIG. 1 is a block diagram of an exemplary hybrid OFDMA system 10including a transmitter 100 and a receiver 200 in accordance with theteachings herein. The transmitter 100 includes a spread OFDMAsubassembly 130, a non-spread OFDMA subassembly 140 and a commonsubassembly 150. In the spread OFDMA subassembly 130, input data 101(for one or more users) is spread with a spreading code to generate aplurality of chips 103 and the chips 103 are then mapped to subcarriers.In the non-spread OFDMA subassembly 140, input bit 111 (for one or moredifferent users) is mapped to subcarriers without spreading.

The spread OFDMA subassembly 130 includes a spreader 102 and a firstsubcarrier mapping unit 104. The non-spread OFDMA subassembly 140includes a serial-to-parallel (S/P) converter 112 and a secondsubcarrier mapping unit 114. The common subassembly 150 includes anN-point inverse discrete Fourier transform (IDFT) processor 122, aparallel-to-serial (P/S) converter 124 and a cyclic prefix (CP)insertion unit 126.

Assuming that there are N subcarriers in the system and that K differentusers communicate at the same time in the system, among K users, data toK_(S) users is transmitted via the spread OFDMA subassembly 130. Thenumber of subcarriers used in the spread OFDMA subassembly 130 and thenon-spread OFDMA subassembly 140 are N_(S) and N_(O), respectively. Thevalues of N_(S) and N_(O) satisfy the conditions that 0≦N_(S)≦N,0≦N_(O)≦N, and N_(S)+N_(O)≦N.

The input data 101 is spread by the spreader 102 to a plurality of chips103. The chips 103 are mapped to the N_(S) subcarriers by the subcarriermapping unit 104. The spreading may be performed in the time domain, inthe frequency domain, or both. For a particular user, spreading factorsin the time domain and the frequency domain are denoted by SF_(t) andSF_(f), respectively. A joint spreading factor for the user is denotedby SF_(joint), which equals to SF_(t)×SF_(f). When SF_(t)=1, thespreading is performed only in the frequency domain, and when SF_(f)=1,the spreading is performed only in the time domain. A frequency domainspreading for user i is limited to the number of subcarriers allocatedto the user i, N_(S)(i). The allocation of subcarriers can be static ordynamic. In the case where N_(S)(i)=N_(S) for every user i, the spreadOFDMA becomes spread OFDM.

One subcarrier may be mapped to more than one user in the spread OFDMAsubassembly 130. In such case input data 101 of two or more users mappedto the same subcarrier are code multiplexed, and therefore, should bespread using different spreading codes. If spreading is performed bothin the time and frequency domain, spreading codes assigned to users maybe different in the time domain, in the frequency domain, or both.

FIG. 2 shows an example of frequency domain spreading and subcarriermapping in accordance with the teachings herein. The input data 101 ismultiplied with a spreading code 204 by a multiplier 202 to generate aplurality of chips 103′. The chips 103′ are converted to parallel chips103 by an S/P converter 206. Each of the parallel chips 103 is thenmapped to one of the subcarriers by the subcarrier mapping unit 104before being sent to the IDFT processor 122.

FIG. 3 shows another example of frequency domain spreading andsubcarrier mapping in accordance with the teachings herein. Instead ofmultiplying a spreading code by a spreader, a repeater 302 may be usedto repeat each input data 101 multiple times at the chip rate togenerate chips 103′. The chips 103′ are then converted to parallel chips103 by an S/P converter 304. Each of the parallel chips 103 is mapped toone of the subcarriers by the subcarrier mapping unit 104 before beingsent to the IDFT processor 122.

Alternatively, when input data is spread in the time domain, each inputdata is spread by a spreader to generate a plurality of chip streams andthe chip streams are mapped to subcarriers. In such case, the timedomain spreading may also be performed by simple repetition of the inputdata without using a spreading code.

Common pilots may be transmitted on the subcarriers used in the spreadOFDMA subassembly 130. In order to distinguish from other user data,common pilots are also spread.

Referring again to FIG. 1, in the non-spread OFDMA subassembly 140,input bits 111 of different users are converted to parallel bits 113 bythe S/P converter 112. The subcarrier mapping unit 114 allocates usersto one or more subcarriers, such that each subcarrier is used by at mostone user and bits from each user are mapped to the allocated subcarriersfor the user by the subcarrier mapping unit. In this way, users aremultiplexed in the frequency domain. The number of subcarriers allocatedto user i is denoted by N_(O)(i), 0≦N_(O)(i)≦N_(O). The allocation ofsubcarriers can be static or dynamic.

In accordance with the teachings herein, time-frequency hopping may beperformed for the non-spread OFDMA subassembly 140 in a pseudo-randomway in each cell. With time domain hopping, the users that transmit in acell change from time to time (i.e., over one or several OFDM symbols orframes). With frequency domain hopping, subcarriers allocated to usersthat transmit in a cell are hopping per one or several OFDM symbols orframes. In this way, the inter-cell interference can be mitigated andaveraged among the users and cells.

FIG. 4 illustrates an example of time-frequency hopping where ten (10)subcarriers, s0-s9, are used for time periods of T0-T6 in accordancewith the teachings herein. As an example, in FIG. 2, subcarriers s3, s5,s8 are used for spread OFDMA and the remaining subcarriers are used fornon-spread OFDMA. For the subcarriers allocated for non-spread OFDMA,subcarriers and time periods allocated to users are hopping in apseudo-random way. For example, data for user 1 is transmitted via s9 atT0, s7 at T1, s7 at T3, and s1 and s9 at T4, and data for user 2 istransmitted via s4 at T0, s6 at T1, s3 at T2, s0 and s4 at T4.Therefore, data to different users is transmitted over different OFDMsymbols or frames and inter-cell interference is mitigated.

Referring again to FIG. 1, both the chips 105 and the data 115 are fedinto the IDFT processor 122. The IDFT processor 122 converts the chips105 and data 115 to time domain data 123. The IDFT may be implemented byIFFT or an equivalent operation. The time domain data 123 is thenconverted to a serial data 125 by the P/S converter 124. A CP (alsoknown as a guard period (GP)) is then added to the serial data 125 bythe CP insertion unit 126. Data 127 is then transmitted via the wirelesschannel 160.

The receiver 200 includes a spread OFDMA subassembly 230, a non-spreadOFDMA subassembly 240 and a common subassembly 250 for hybrid OFDMA. Thecommon subassembly 250 includes a CP removal unit 202, a P/S converter204, an N-point discrete Fourier transform (DFT) processor 206, anequalizer 208 and a subcarrier demapping unit 210. The spread OFDMAsubassembly 230 includes a code domain user separation unit 214 and thenon-spread OFDMA subassembly 240 includes a P/S converter 216.

The receiver 200 receives data 201 transmitted via the channel, A CP isremoved from received data 201 by the CP removal unit 202. Data 203after the CP is removed, which is time domain data, is converted toparallel data 205 by the S/P converter 204. The parallel data 205 is fedto the DFT processor 206 and converted to frequency domain data 207,which means N parallel data on N subcarriers. The DFT may be implementedby FFT or equivalent operation. The frequency domain data 207 is fed tothe equalizer 208 and equalization is performed to data at eachsubcarrier. As in a conventional OFDM system, a simple one-tap equalizermay be used.

After equalization at each subcarrier, data corresponding to aparticular user is separated by the subcarrier demapping unit 210, whichis an opposite operation performed by the subcarrier mapping units 104,114 at the transmitter 100. In the non-spread OFDMA subassembly 240,each user data 211 is simply converted to a serial data 217 by the S/Pconverter 216. In the spread OFDMA subassembly 230, data 212 on theseparated subcarriers are further processed by the code domain userseparation unit 214. Depending on the way spreading is performed at thetransmitter 100 corresponding user separation is performed in the codedomain user separation unit 214. For example, if the spreading isperformed only in the time domain at the transmitter 100, a conventionalRake combiner may be used as the code domain user separation unit 214.If the spreading is performed only in the frequency domain at thetransmitter 100, a conventional (frequency domain) despreader may beused as the code domain user separation unit 214. If the spreading isperformed in both the time domain and the frequency domain at thetransmitter 100, a time-frequency Rake combiner may be used as the codedomain user separation unit 214.

FIG. 5 is a block diagram of an exemplary time-frequency Rake combiner500 configured in accordance with the teachings herein. Thetime-frequency Rake combiner 500 performs processing at both time andfrequency domains in order to recover data that is spread in both timeand frequency domains at the transmitter 100. It should be noted thatthe time-frequency Rake combiners 500 may be implemented in manydifferent ways and the configuration shown in FIG. 5 is provided as anexample, not as a limitation, and the scope of the teachings herein isnot limited to the structure shown in FIG. 5.

The time-frequency Rake combiner 500 comprises a despreader 502 and aRake combiner 504. Data 212 separated and collected for a particularuser by the subcarrier demapping unit 210 in FIG. 1 for the spread OFDMAsubassembly 230 is forwarded to the despreader 502. The despreader 502performs frequency-domain despreading to the data 212 on thesubcarriers. The despreader 502 includes a plurality of multipliers 506for multiplying conjugate 508 of the spreading codes to the data 212, asummer 512 for summing the multiplication outputs 510, and a normalizer516 for normalizing the summed output 514. The despreader output 518 isthen processed by the Rake combiner 504 to recover the data of the userby time domain combining.

Referring again to FIG. 1, the transmitter 100, the receiver 200, orboth may include multiple antennas and may implement hybrid OFDMA inaccordance with the teachings herein with multiple antennas either attransmitter side, the receiver side, or both.

Although the features and elements herein are described in the preferredembodiments in particular combinations, each feature or element can beused alone without the other features and elements of the preferredembodiments or in various combinations with or without other featuresand elements described herein.

1. A wireless transmit/receive unit (WTRU) comprising: a receiverconfigured to receive in a first single symbol time interval a pluralityof subcarriers; wherein a first group of subcarriers from the pluralityof subcarriers includes information for the WTRU combined with anorthogonal sequence; wherein the first group of subcarriers includesinformation for a plurality of WTRUs; wherein the information for eachof the plurality of WTRUs is combined with an orthogonal sequence andthe orthogonal sequence for each of the plurality of WTRUs is different;wherein a second group of subcarriers from the plurality of subcarriersincludes information for the WTRU not combined with an orthogonalsequence; wherein the receiver is further configured to recover theinformation for the WTRU from the first group of subcarriers and thereceiver is further configured to recover the information for the WTRUfrom the second group of subcarriers; and wherein a third group ofsubcarriers from the plurality of subcarriers in the first single symboltime interval includes information for another WTRU that is not combinedwith an orthogonal sequence.
 2. The WTRU of claim 1 wherein the firstgroup of subcarriers are not contiguous subcarriers.
 3. The WTRU ofclaim 1 wherein the receiver is configured to recover the informationfrom the first group of subcarriers using despreading.
 4. The WTRU ofclaim 1 wherein the information in the first group of subcarriers istime repeated in a second single symbol time interval and the orthogonalsequence in the first single symbol time interval and the second singlesymbol time interval frequency spreads the information; wherein thereceiver is further configured to recover the information using the timerepeated information.
 5. The WTRU of claim 1 wherein the first singlesymbol time interval includes pilot signals.
 6. The WTRU of claim 5wherein the WTRU uses the pilot signals to aid in recovering theinformation from the first and second groups of subcarriers.
 7. A methodcomprising: receiving by a wireless transmit/receive unit (WTRU) in afirst single symbol time interval a plurality of subcarriers; wherein afirst group of subcarriers from the plurality of subcarriers includesinformation for the WTRU combined with an orthogonal sequence; whereinthe first group of subcarriers includes information for a plurality ofWTRUs; wherein the information for each of the plurality of WTRUs iscombined with an orthogonal sequence and the orthogonal sequence foreach of the plurality of WTRUs is different; wherein a second group ofsubcarriers from the plurality of subcarriers includes information forthe WTRU not combined with an orthogonal sequence; recovering by theWTRU the information for the WTRU from the first group of subcarriers;recovering by the WTRU the information for the WTRU from the secondgroup of subcarriers; and wherein a third group of subcarriers from theplurality of subcarriers in the first single symbol time intervalincludes information for another WTRU that is not combined with anorthogonal sequence.
 8. The method of claim 7 wherein the first group ofsubcarriers are not contiguous subcarriers.
 9. The method of claim 7wherein the information recovered from the first group of subcarriers isperformed using despreading.
 10. The method of claim 7 wherein theinformation in the first group of subcarriers is time repeated in asecond single symbol time interval and the orthogonal sequence in thefirst single symbol time interval and the second single symbol timeinterval frequency spreads the information; wherein the recovering theinformation includes using the time repeated information.
 11. The methodof claim 7 wherein the first single symbol time interval includes pilotsignals.
 12. The method of claim 11 wherein the WTRU uses the pilotsignals to aid in recovering the information from the first and secondgroups of subcarriers.
 13. A wireless network device comprising: atransmitter configured to transmit in a first single symbol timeinterval a plurality of subcarriers; wherein a first group ofsubcarriers from the plurality of subcarriers includes information for afirst wireless transmit/receive unit (WTRU) combined with an orthogonalsequence; wherein the first group of subcarriers includes informationfor a plurality of WTRUs; wherein the information for each of theplurality of WTRUs is combined with an orthogonal sequence and theorthogonal sequence for each of the plurality of WTRUs is different;wherein a second group of subcarriers from the plurality of subcarriersincludes information for the first WTRU not combined with an orthogonalsequence; wherein a third group of subcarriers from the plurality ofsubcarriers in the first single symbol time interval includesinformation for a second WTRU that is not combined with an orthogonalsequence; wherein the first WTRU and the second WTRU are different. 14.The wireless network device of claim 13 wherein the first group ofsubcarriers are not contiguous subcarriers.
 15. The wireless networkdevice of claim 13 wherein the information in the first group ofsubcarriers is time repeated in a second single symbol time interval andthe orthogonal sequence in the first single symbol time interval and thesecond single symbol time interval frequency spreads the information.16. The wireless network device of claim 13 further comprisingtransmitting pilot signals in the first single symbol time interval.